1. Field of the Invention
The present invention relates to a support sheet for a photographic printing sheet, more particularly, the present invention relates to a support sheet for a photographic printing sheet, having an electron beam-cured resin layer, exhibiting an excellent resistance to yellowing thereof by the developing treatment and having sufficient flexibility.
2. Description of the Related Art
Currently, a polyolefin-coated support sheet, produced by coating both surfaces of a substrate, consisting of a paper sheet, with a polyolefin resin, is widely employed as a support sheet for photographic printing paper. This type of support sheet is advantageous in that in a develop-fixing treatment for a resultant photographic printing paper sheet, since the treating liquid does not penetrate into the support sheet, the shrinkage and elongation of the support sheet are restricted and thus the support sheet exhibits an excellent dimensional stability.
For the purpose of enhancing the opacity or the image-forming properties, the polyolefin resin in the coating layer of the support sheet is mixed with an inorganic white pigment such as titanium dioxide. The pigment has a poor dispersing property in the resin and causes a problem in that when the resin is melt-extruded, the resin is foamed by a volatile component contained in the pigment and thus the resultant resin coating layer is cracked or broken. For this reason, it is impossible to increase the content of the pigment in the coating layer to a level sufficient to obtain a satisfactory opacity and suitable image-forming properties.
Generally speaking, when titanium dioxide pigment is employed, it is difficult to add this pigment at a rate of more than about 20% by weight. Therefore, the photographic printing paper produced from this type of support sheet for photographic printing sheet is unsatisfactory in the sharpness of the image.
Recently, a support sheet for photographic printing paper, having an electron-beam-cured resin-coated layer formed by coating a resin composition curable by electron beam irradiation, namely an electron-beam-curable resin, on a support sheet, was provided by, for example, Japanese Examined Patent Publication (Kokoku) No. 60-17,104, Japanese Examined Patent Publication (Kokoku) No. 60-17,105, and Japanese Unexamined Patent Publication (Kokai) No. 57-49,496. In accordance with this method, when the coating layer is formed, it is unnecessary to heat and melt the resin composition at a high temperature, and thus the pigment content can increase from 20 to 80% by weight. Therefore, the photographic printing paper sheet produced with the above-mentioned type of support sheet has a significantly enhanced image sharpness in comparison with the conventional photographic printing paper sheet having polyolefin resin coating layers. Nevertheless, with respect to the photographic printing paper produced by coating a photographic photosensitive layer on an electron-beam-cured resin-coating layer cured by an electron-beam irradiation, it is known that in the developing treatment, a phenomenon that a photographic developing reagent is absorbed by and remains in the resin coating layer, and thus the photographic printing sheet is discolored yellow after the developing treatment, namely a yellowing phenomenon occurs, and when a developing treatment is applied to the photographic printing paper after a storage period, the degree of fogging can increase to a level not negligible for practical use and the photosensitivity thereof can change. It is also known that the resultant coating layer is hard and has little flexibility and thus is sometimes cracked or broken.
There have been made various attempts to prevent the above-mentioned yellowing problem and to make the electron-beam-cured resin layer flexible. In these attempts specific electron-beam-curable polymers or electron-beam-curable monomers are employed. For example, Japanese Unexamined Patent Publication (Kokai) No. 59-124,336 discloses specific electron-beam-curable acrylic acid ester monomers, Japanese Unexamined Patent Publication (Kokai) No. 60-70,446 discloses specific urethane resins having double bonds. Japanese Unexamined Patent Publication (Kokai) No. 61-201,241 discloses specific diacrylates, triacrylates and expoxidized acrylates. Japanese Unexamined Patent Publication (Kokai) No. 61-236,547 discloses specific tetraacrylic acid esters. Japanese Unexamined Patent Publication (Kokai) No. 62-61,049 discloses specific hexaacrylate esters. Japanese Unexamined Patent Publication (Kokai) No. 62-109,046 discloses specific polybutadiene-containing resins. Japanese Unexamined Patent Publication (Kokai) No. 2-47 discloses specific acrylate esters of addition products of acrylic acid polymers with polyhydric alcohols.
These specific electron-beam-curable compounds do not, however, completely solve the above-mentioned problems.
It is known that these is a close relationship between the yellowing phenomenon and the flexibility of the electron-beam-cured resin and those properties are not proportional with the exposed dose of the electron-beam. Namely, when the electron-beam is irradiated at a high exposure dose, the resultant cured resin layer exhibits a reduced flexibility although the yellowing phenomenon thereof is restricted. Also, the high dose of the electron-beam causes the substrate paper sheet to exhibit a reduced mechanical strength and a deteriorated quality. Also, when the electron beam is irradiated at a low exposure dose, the resultant cured resin layer exhibits a significantly increased yellowing phenomenon, although the flexibility of the cured resin layer is secured to a certain extent. Also, the cured resin layer exhibits reduced adhesive property and mechanical strength.
Accordingly, to prevent the yellowing phenomenon without reducing the physical properties of the cured resin layer, it is necessary to provide a structure of a new support sheet in which an appropriately selected electron-beam-curable unsaturated organic compound is used, and even when the electron-beam is applied in an exposure dose large enough to bring about the formation of crosslinkages in the cured resin layer, the flexibility of the cured resin layer and the mechanical properties and quality of the substrate paper sheet are not deteriorated. It is very important to effectively eliminate all the above-mentioned problems simultaneously.